1 Field of the Invention
The present invention relates to polyorganosiloxane compositions crosslinkable by addition of Si-bonded hydrogen to aliphatic carbon-carbon multiple bonds, to processes for their preparation, to platinum catalysts used for this purpose, to the use of the crosslinkable polyorganosiloxane compositions in a process for preparing silicone elastomers, and to the use of the silicone elastomers thusly obtained.
2. Background Art
Addition-crosslinking polyorganosiloxane compositions crosslink by reaction of aliphatically unsaturated groups with Si-bonded hydrogen (hydrosilylation) in the presence of a catalyst, typically a platinum compound. Owing to the fact that the crosslinking reaction begins when simultaneous presence of the essential constituents is established, addition-crosslinking polyorganosiloxane compositions have to date been prepared virtually exclusively as two-component formulations, the composition of the individual components being such that only when they are mixed are all three essential constituents present at the same time. Typically, one of the components contains the alkenyl-functional polyorganosiloxane and the platinum catalyst, the other component the SiH-functional crosslinker, optionally in combination with further alkenyl-functional polyorganosiloxane. After the individual components have been mixed, full curing to the silicone elastomer can be effected at room temperature, but is typically carried out at elevated temperature.
Two-component systems of addition-crosslinkable polyorganosiloxane compositions are associated with numerous disadvantages, for instance, logistics, the high risk of contamination by traces of platinum, and the necessity of an additional mixing step. After the components have been mixed, a ready-to-use composition is obtained, but this composition has only a highly restricted pot life at room temperature. This restricted pot life necessitates that processing or use follow immediately, and also requires frequent cleaning of the stock vessel, metering units or processing machines, etc., since any material remaining, for example by virtue of backmixing or wall adhesion, will gel.
Owing to the disadvantages mentioned, there has been no shortage of attempts to provide addition-crosslinking polyorganosiloxane compositions as a one-component formulation (1K system). Since, in the case of a 1K system, all constituents needed for crosslinking are present together, the fundamental problem consists of suppressing premature crosslinking, which normally proceeds even at room temperature. Means of controlled adjustment, generally prolonging of the pot life of an addition-crosslinking composition, are well known, for example the use of inhibitors capable of heavily decreasing the activity of the platinum catalyst at room temperature. Examples of such inhibitors include, for example, phosphorus compounds in combination with peroxides according to U.S. Pat. No. 4,329,275 A, or azodicarbonyl compounds, as described, for example, in European Laid-Open Specification EP 0 490 523 A1. Although it is possible to prolong the pot life per se as much as desired by the type and content of such inhibitors, a disadvantageous influence of the crosslinking performance is inseparably associated with increasing pot life. This is especially true when the pot life is extended to several months by high inhibitor contents, which result in the consequence of an increased curing onset temperature with simultaneously lower crosslinking rate, to the extent of under-crosslinking in many compositions.
A further, fundamentally different approach consists in encapsulating the platinum catalyst in a finely divided material which releases the platinum only at elevated temperature. This can be done, for example, by microencapsulation of the platinum catalyst with a thermoplastic silicone resin or an organic thermoplastic, as described, for instance, in the European Laid-Open Specification EP 0 363 006 A1. Owing to the preparation of the microencapsulated catalysts, this approach is very costly and inconvenient. The resin must not dissolve in the silicone rubber, since the catalyst would otherwise escape and the pot life would be drastically lowered. Furthermore, the microencapsulated catalyst is heterogeneously distributed in the silicone, which has an adverse effect on the crosslinking, since the polyorganosiloxane composition cannot thus crosslink homogeneously. A further approach to extending pot life is the inclusion of the catalyst in a cyclodextrin, as described, for example, in European Patents EP 0 423 588 B1 and EP 0 491 509 B1. However, this approach has the great disadvantage that the inclusion compounds are insoluble in silicone and thus there is again no homogeneous distribution in the silicone. As a result, the crosslinking time is very slow even at elevated temperatures; these crosslinkable polyorganosiloxane compositions are unsuitable for processing in injection molding. A further approach employs specific platinum complexes whose activity is “latent” such that, although the hydrosilylation reaction proceeds rapidly at elevated temperature, at room temperature, the reaction rate is low enough that pot lives of several months are achieved. Polyorganosiloxane compositions which addition-crosslink with initiation by means of UV radiation and comprise platinum complexes have been described, for example, in the Japanese Laid-Open Specification JP09-040870 A2 and in the German Laid-Open Specification DE 36 35 236 A1. Further platinum complexes are disclosed in the Patents EP 0 982 370 A1, EP 0 994 159 A1 and EP 1 077 226 A1.
Even though the compositions described, especially those mentioned in the last two patents, have distinctly improved pot lives with sometimes sufficiently high crosslinking rates, there is still a need to improve the pot life and crosslinking rate of addition-crosslinking polyorganosiloxane compositions, especially those with a one-component formulation, by higher-performance platinum catalysts, without having to accept the disadvantages of the prior art mentioned above. In particular, there is a need for compositions with lower onset temperatures, but nevertheless long pot lives at room temperature.